Chinese Patent Application No. 94100421.x, entitled “Internal Combustion Engine having Cam Piston” and published on Dec. 28, 1994, discloses a pin shaft (roller shaft) comprising a suspension beam having a free end received in an internal groove of a cam, which must be provided with a piston in order to balance the other end of the beam, as shown in FIGS. 1, 4 and 5. Therefore, the piston exerts a lateral force to the cylinder, which deteriorates the service life of the piston and the cylinder. Since the projection image of the upper circumferential cam contour completely overlaps with that of the lower circumferential cam contour, the roller generates inverse torque when the pin shaft bearing impacts from one side to the other side across the groove of the circumferential cam contour, resulting serious wearing at the cross point. In addition, it is extremely inconvenient to replace the rollers and to design a single-cylinder internal combustion engine and a multi-cylinder internal combustion engine by simply combining cylinders.